The present invention relates to a steering torque detecting apparatus for use in an electrically-operated power steering system of a vehicle such as an automobile. More particularly, the present invention pertains to an improved steering torque detecting apparatus of this type which is designed to prevent generation of an erroneous detection signal due to flexural load applied between a pair of input and output shafts and to an improved steering torque detecting apparatus wherein a mounting ring for mounting a slider and a skip ring part having a printed board mounted thereon are movably coupled to each other so that storage and transportation of these members are facilitated. The present invention is also concerned with an improvement in a slip ring part which is fitted on a pair of input and output shafts in a steering torque detecting apparatus of the type described above.
FIG. 16 is a longitudinal sectional view of the conventional steering torque detecting apparatus disclosed in Japanese Utility Model Application No. 62-91811 (1987). In FIG. 16, reference numerals 1 and 2 denote input and output shafts, respectively, which constitute in combination a steering shaft, while numeral 3 denotes a torsion bar which couples together the input and output shafts 1 and 2, and numeral 4 denotes a fixing pin. Reference numeral 6 denotes a first housing which is supported by a stationary part (not shown) and which supports the input shaft 1 through a bearing 8, and numeral 7 a second housing which is coupled to the housing 6 by means of a securing screw 10 and which supports the output shaft 2 through a bearing 9.
Reference numeral 11 denotes a support ring for the slip ring which is molded from a synthetic resin material and which is rigidly secured to the input shaft 1, the support ring 11 being provided with a flange portion 11a. Reference numeral 12 denotes a plurality of slip rings buried in the support ring 11. Connecting wires 13 are led out from the respective slip rings 12.
Reference numeral 14 denotes a disc-shaped printed board which is secured to the flange portion 11a. The printed board 14 has a potentiometer element formed on one side thereof which is provided with a circumferential resistance layer and a plurality of electrodes. Reference numeral 17 denotes a mounting ring which is made of an insulating material and secured by means of a mounting screw 18, while numeral 16 denotes a slider which is secured to the mounting ring 17 so as to extend axially, the slider 16 being in contact with both the resistance layer and electrodes of the potentiometer element 15.
Reference numeral 19 denotes a brush device which is arranged as follows. Reference numeral 20 denotes a brush holder which is made of an insulating material and is secured to the housing 6. Numeral 21 denotes brushes made of resilient thin metallic wires having good electrical conductivity. The distal ends of the brushes 21 are in tangential contact with the respective slip rings 12, while the proximal ends of the brushes 21 are rigidly secured to respective terminals 22. The terminals 22 are buried in the brush holder 20 and connected to lead wires 24 through capacitors 23. Reference numeral 25 denotes a cover.
The above-described conventional torque detecting apparatus operates as follows. When no steering operation is being conducted, no torsion is generated in the torsion bar 3 and there is therefore no change in the output from the potentiometer that detects the level of torque.
When the steering wheel is turned, the torsion bar 3 is torsionally deformed and the position of contact of the slider 16 with the resistance layer is displaced from a neutral point in proportion to the amount of torsional deflection of the torsion bar 3, and an output signal which is proportional to the amount of displacement of the contact position of the slider 16 is thus obtained from the potentiometer. In accordance with the amount and direction of torque detected in this way, an electric motor for the steering operation is driven to assist the output shaft 2 to rotate in the detected direction.
The conventional steering torque detecting apparatus of the type described above suffers, however, from the following problems. Namely, not only torsion but also flexural load is transmitted from the input shaft 1 to the output shaft 2 and therefore the amount of displacement of the contact position of the slider 16 with respect to the resistance layer of the potentiometer element 15 which corresponds to the level of the flexural load is also included in the output. Therefore, the output signal represents a higher level of torque than what is actually being applied. Accordingly, the driving output from the steering motor becomes excessively high, resulting in a greater amount of assistance being given than the amount required by the driver turning the steering wheel.
Further, when the steering wheel is turned, torsion is generated in the torsion bar 3 and flexural load is also applied between the input and output shafts 1 and 2. The flexural load causes a change in the parallel relationship between the surface of the printed board 14 and the surface of the mounting ring 17 which faces it and the position of contact of the slider 16 with the resistance layer of the potentiometer element 15 is therefore displaced, which results in an erroneous detection signal that includes an error in addition to a signal representative of the level of the actually applied torque. Generation of such an erroneous detection signal obstructs achievement of stable steering.
In the above-described conventional steering torque detecting apparatus, the mounting ring having the slider mounted thereon and the slip ring part having the printed board secured thereto are combined together as a pair of parts. However, these two parts are stored and transported as separate structures and there are therefore problems in handling, management and transportation of the two parts. More specifically, foreign matter is likely to adhere to the slider and the resistance layer provided on the surface of the potentiometer element formed on the printed board during storage and transportation of the parts, and a great deal of time and labor is needed to pack the parts. In addition, since the mounting ring and the printed board are separate from each other even after they have been assembled to the input and output shafts, there is a risk of foreign matter, e.g., dust or oil for bearing, entering the area of contact between the slider and the resistance layer.
In the prior art apparatus, the slip ring supporting ring 11 is molded from a thermosetting synthetic resin material and press-fitted on the input shaft 1. Therefore, if the degree of interference is excessive, the support ring 11 may be cracked, whereas, if the degree of interference is too small, the fitting engagement between the support ring 11 and the input shaft 1 will be loose. Accordingly, it is necessary to conduct machining with a precise tolerance in order to obtain an optimal level of interference at which no looseness will be produced even if there is thermal expansion or contraction. If the degree of interference is excessive, the outer diameter of the support ring 11 is increased when press-fitted on the input shaft 1, which results in the slip rings 12 being subjected to stress and therefore becoming liable to crack. Further, the circumferential position of the resistance layer formed on the printed board 14 needs to be set at a predetermined position with respect to the input shaft 1. Since the support ring 11 is simply press-fitted on the input shaft 1, although the printed board 14 is mounted on the flange 11a while being circumferentially positioned with respect to it, it is necessary to conduct high precision assembly so that the circumferential position of the resistance layer is set in a predetermined position with respect to the input shaft 1 when the support ring 11 is press-fitted on the shaft 1.